• The team led by Claudia Gutiérrez from the ICE-CSIC and IEEC has used the Gran Telescopio Canarias (GTC) and the Nordic Optical Telescope (NOT), at the Roque de Los Muchachos Observatory, of the Instituto de Astrofísica de Canarias (IAC), in La Palma.

• The CSS161010 burst reached its maximum brightness in just 4 days in a small galaxy 500 million light-years away from us.

An international scientific team, led by the Institute of Space Studies of Catalonia (IEEC) and the Institute of Space Sciences (ICE-CSIC), has managed to detect an exceptionally fast and bright cosmic burst in a small galaxy located 500 million light years away. This discovery is published today in a study in The Astrophysical Journal.

The burst, identified as CSS161010, reached its maximum brightness in just 4 days and dropped to half in just 2.5 days, which meant that both its discovery and the subsequent observations of its evolution became a scientific milestone and a challenge for the research team. The main author of this work is Dr. Claudia Gutiérrez, a researcher at the IEEC and ICE-CSIC.

The CSS161010 event was discovered by the Catalina Real-Time Transient Survey, with a previous detection reported by the All-Sky Automated Survey for SuperNovae. Its subsequent follow-up, which allowed its characterization, was carried out with Telescopes including the Gran Telescopio Canarias (GTC) and the Nordic Optical Telescope (NOT), both installed at the Roque de Los Muchachos Observatory, of the Instituto de Astrofísica de Canarias (IAC), located in the municipality of Garafía in La Palma.

These types of rapidly evolving cosmic phenomena have been very difficult to study due to their nature. However, modern techniques and more advanced instruments make it possible to study them thanks to the improved field of view and the ability to capture high-resolution images of the telescopes used.

To date, only a dozen cosmic explosions with these characteristics in terms of brightness and evolution have been detected, but their origin remains a complete mystery. However, the team of researchers led by Claudia Gutiérrez believes that, for the first time, the unique spectral properties of CSS161010 provide important clues about its physical origin and their analysis suggests that it is actually resulting from a small black hole swallowing a star.

This conclusion was reached as the team found broad hydrogen lines showing very high speed, up to 10% of the light speed, and an unprecedented evolution. Two months after the start of the outburst, the object's brightness had decreased 900 times compared to its maximum. Surprisingly, the spectra captured by the Gran Telescopio Canarias at this time revealed that all the hydrogen line profiles were still blueshifted, which in astrophysics means they are moving towards us at extremely high speeds. This would indicate a strong gas outflow, something completely unforeseen for a supernova.

“Discovering and analysing these cosmic explosions is particularly challenging due to their rapid evolution. However, the agile response of our scientific collaboration allowed us to obtain high-quality spectra. These data revealed unique properties never observed in any other object, allowing us to constrain the nature of this extraordinary event,” explains postdoctoral researcher Claudia Gutiérrez.

“When we saw the spectra, we didn’t know what to say,” says Gutiérrez. “We had never found a hydrogen line profile so blueshifted; this shift would mean that the gas is moving towards us at extremely high speeds. This feature was both surprising and intriguing, prompting us to investigate possible connections with the galaxy where the event occurred,” she adds.

Looking for intermediate mass black holes

The burst occurred in a tiny galaxy containing a mass of stars about 400 times less than our Milky Way’s. Therefore, if the galaxy hosts a massive black hole, its mass must also be small, corresponding to an intermediate-mass black hole (100 - 100.000 solar masses).

“So far, this kind of black holes have been extremely hard to identify and astronomers are only aware of a very small number of confirmed cases”, explains professor Seppo Mattila, from the University of Turku in Finland, one of the lead authors of the paper.

“Identifying and characterising intermediate-mass black holes is essential for understanding black hole formation pathways and evolution. In fact, they are the fundamental building blocks of supermassive black holes found at the centre of galaxies, such as our Milky Way, and  observed to exist even in the early Universe,” adds Professor Mattila.

Professor Peter Lundqvist from Stockholm University, also part of this team, adds: “The way the line emission evolves in this object resembles the one observed in active galactic nuclei, where supermassive black holes are known to exist. This similarity provides strong evidence that CSS161010 also hosts a black hole, although not a very massive one.”

Lundqvist points out: “The disruption of a star that came too close to the intermediate-mass black hole reveals the black hole, which would be quiescent otherwise. There are likely to be other such black holes in other dwarf galaxies, and we need to track down events similar to CSS161010 in order to determine the properties of these black holes more precisely.”

“Telescopes that scan the sky at high cadence will be crucial to discover more of these rare and rapidly evolving phenomena,” says Dr. Gutiérrez, adding: “In the meantime, state-of-the-art spectrographs on ground-based telescopes such as those we have used at the Roque de los Muchachos Observatory in La Palma will play a crucial role in characterising them. We are at the cusp of an era of ground-breaking discoveries.”

More information

C. P. Gutiérrez et al. (2024), CSS161010: A Luminous, Fast Blue Optical Transient with Broad Blueshifted Hydrogen Lines, ApJ, vol. 977. DOI:10.3847/1538-4357/ad89a5